The present invention relates generally to exhaust gas recirculation in an internal combustion engine and more particularly to an improvement in an EGR (Exhaust gas Recirculation) control system for controlling recirculation of exhaust gases back to the intake.
It is well known in the art that a portion of exhaust gases emitted from an internal combustion engine is recirculated back to an intake system of the engine for thereby reducing formation of oxides of nitrogen during the combustion process in the engine. However, since this exhaust gas recirculation considerably effects the combustion in the engine and consequently the operating efficiency of the engine, the exhaust gas recirculation must be strictly controlled in respect to the operating conditions of the engine. In general, for the purpose of achieving efficient reduction in formation of oxides of nitrogen without impairing the operating efficiency or the power output of the engine, it is desired to recirculate the exhaust gases at a rate proportional to the rate at which combustion air flows into the engine, viz., it is desired to maintain at a predetermined or constant value the EGR ratio, which is the ratio of the recirculated amount of exhaust gases to the amount of air inducted to the engine, by weight. On the other hand, for the purpose of improving the fuel consumption of the engine, it is desired to reduce the EGR ratio below the above-mentioned predetermined value during particular operating conditions of the engine, e.g., during high speed low load operating conditions of the engine. Accordingly, it is necessary to control exhaust gas recirculation with enough consideration for the operating efficiency of the power output and the fuel consumption of the engine.
As an expedient for attaining this purpose, an EGR control system has been proposed by the same applicant as this application, which system is disclosed in U.S. Pat. application No. 817,994, filed July 21, 1977. For explanation of the background of this invention, this EGR control system is shown in FIG. 1 of the accompanying drawings of this application.
In FIG. 1, designated by the reference numeral 10 is an induction passage of an internal combustion engine (not shown), by 12 a venturi formed in the induction passage, and by 14 a throttle in the form of a rotatable throttle valve disposed in the induction passage 10 for controlling air flow to the engine. This EGR control system comprises an EGR passage 16 interconnecting an exhaust passage (not shown) and the induction passage 10 downstream of the throttle 14, a restriction or an orifice 18 formed in the EGR passage 16 for restriction of same and an EGR control valve 20 disposed in the EGR passage 16 downstream of the restriction 18. The EGR control valve 20 includes a flexible diaphragm 22, and first and second fluid chambers 23 and 24 separated by the diaphragm 22 from each other. The fluid chamber 23 communicates with the induction passage 10 downstream of the throttle 14 through a passage 25 for receiving therein a suction vacuum, and the fluid chamber 24 communicates with the atmosphere through a port 24'. A valve stem 26 is fixedly connected at its one end to the diaphragm 22 so as to be movable therewith as one body. A valve head 27 is integrally connected to the other end of the valve stem 26 and is sealingly seatable on a valve seat 28 disposed in the EGR passage 16 downstream of the restriction 18. A spring 29 is disposed in the fluid chamber 23 for urging the diaphragm 22 in the direction of the valve seat 28. By the EGR control valve 20 thus constructed and arranged as above, the EGR passage 16 is provided with a chamber or a passage zone 16' defined between the restriction 18 and the EGR control valve 20. A pressure regulating valve assembly 30 is provided for controlling the flow of atmospheric air admitted into the fluid chamber 23 for dilution of the suction vacuum therein in accordance with a venturi vacuum in the venturi 12 and the pressure (Pe) in the passage zone 16' defined between the orifice 18 and the EGR control valve 20. For that purpose, the pressure regulating valve assembly 30 includes four chambers 31, 32, 33 and 34, and three diaphragms 36, 38 and 40 respectively separating the above four chambers as shown in the drawing. These three diaphragms 36, 38 and 40 are fixedly connected to each other so that they are operated as one body. The chamber 31 communicates with the atmosphere through openings 42 and further with the passage 25 through a passage 44 which has an inlet port 44' disposed in the chamber 31. The chamber 32 communicates with the venturi 12 through a passage 46 for receiving therein the venturi vacuum. The chamber 33 communicates with the atmosphere through an opening 33'. The chamber 34 communicates with the passage zone 16' of the EGR passage 16 through a passage 47 for receiving therein the pressure (Pe) in the passage zone 16'. The diaphragm 36 is cooperative with the inlet port 44' of the passage 44 to create a valve effect, viz., the diaphragm 36 is movable relative to the inlet port 44' to control the flow of atmospheric air admitted into the passage 44.
The pressure regulating valve assembly 30 thus described briefly is thus operative to increase and reduce the vacuum in the chamber 23 of the EGR control valve 20 in accordance with the venturi vacuum and the pressure (Pe) as follows.
Upon an increase of the venturi vacuum in response to an increase of the amount of air to the engine, the diaphragms 36, 38 and 40 are integrally moved in the direction to close the inlet port 44' to reduce the flow rate of atmospheric air admitted into the passage 44 thus reducing dilution of the vacuum in the chamber 23 of the EGR control valve 20. As a result, the degree of opening of the EGR control valve 20 is increased thus causing decrease of the pressure (Pe) in the passage zone 16' and therefore the pressure in the chamber 34 of the pressure regulating valve assembly 30. When the pressure (Pe) in the passage zone 16' and therefore the pressure in the chamber 34 of the pressure regulating valve assembly 30 are thus reduced, the diaphragms 36, 38 and 40 are, on the contrary, actuated to move in the direction to open the inlet port 44' of the passage 44 to increase the flow rate of atmospheric air into the passage 44. As a result, the dilution of the vacuum in the chamber 23 is increased to reduce the degree of opening of the EGR control valve 20 thus causing an increase of the pressure (Pe) in the passage zone 16'. By the repetition of an operation thus described as above, the degree of opening of the EGR control valve 20 and therefore the pressure (Pe) in the passage zone 16' are converged to the values in which the pressure (Pe) is proportioned to the venturi vacuum so that the flow rate of recirculated exhaust gases is increased and reduced in accordance with increase and decrease of the venturi varuum.
In this EGR control system, the amount of recirculated exhaust gases through the EGR passage 16 is a function of the pressure differential across the restriction 18, viz., upstream and downstream of the restriction 18, and the size of the restriction 18. In this regard, the pressure in the EGR passage upstream of the restriction 18 is approximately equal to the pressure in the exhaust passage (not shown). The exhaust passage pressure is a positive pressure the absolute value of which is relatively small, in other words, the exhaust passage pressure varies within a relatively narrow range through the flow rate of exhaust gases passing through the exhaust passage varies within a relatively wide range in response to the variations of the operating conditions of the engine. Therefore, when the absolute value of the pressure (Pe) in the passage zone 16' is set at a desirable large value as compared to the pressure in the EGR passage upstream of the restriction 18 and is controlled by the EGR control valve 20 in a manner as has been explained hereinbefore, the pressure differential across the restriction 18 is proportioned to the flow rate of combustion air to the engine. Accordingly, the flow rate of recirculated exhaust gases is controlled in proportion to the flow rate of combustion air flowing into the engine.
This EGR control system further has such a function as follows: When the pressure (Pe) in the passage zone 16' changes regardless of the venturi vacuum by the effect of the variations of the suction vacuum in the induction passage 10, the pressure regulating valve assembly 30 operates the EGR control valve 20 to move in the direction to cancel the variations of the pressure (Pe) having been resulted as above. To explain this function of the EGR control system in more detail, when the pressure (Pe) is a negative pressure and this is increased regardless of the venturi vacuum, the diaphragms 36, 38 and 40 are integrally moved in the direction to open the inlet port 44' by the amount corresponding to the increase of the pressure (Pe). As a result, the degree of opening of the EGR control valve 20 is reduced in a manner as has been explained hereinbefore, thus cancelling the influence of the fluctuations of the suction vacuum on the pressure (Pe). The pressure (Pe) is thus restored to its initial value. Accordingly, the flow rate of recirculated exhaust gases is assuredly prevented from being varied irrespective of the venturi vacuum.
A relief valve 48 is provided in the EGR control system and which includes a flexible diaphragm 50 and two fluid chambers 52 and 54 separated by the diaphragm 50 from each other. The fluid chamber 52 communicates with the passage 25 at a location thereof between orifices 56 and 58 through a passage 59. The fluid chamber 54 communicates with the atmosphere through a port 60. A passage 61 is provided which communicates with the passage 46 and has an inlet port 61' located in the chamber 54 for admitting into the passage 61 atmospheric air. The diaphragm 50 is cooperative with the inlet port 61' to create a valve effect, viz., the diaphragm 50 is movable relative to the inlet port 61' between two positions to open and close same to control admission of atmospheric air into the passage 61. A spring 62 is disposed in the chamber 52 for urging the diaphragm 50 in the direction of the inlet port 61', viz., in the direction to close the inlet port 61'. The chamber 52 thus constructed and arranged receives therein two vacuum signals, one of which is a suction vacuum conducted to the chamber 52 through the orifice 56 and the other of which is a vacuum prevailing in the chamber 23 of the EGR control valve and conducted to the chamber 52 through the orifice 58. Accordingly, the vacuum prevailing in the chamber 52 is a vacuum composed of the above two vacuum signals, and the vacuum in the chamber 52 increases and decreases in accordance with a vacuum prevailing in the chamber 23 of the EGR control valve. Since the suction vacuum increases as the load on the engine decreases and the venturi vacuum increases as the RPM of the engine speed increases, the vacuum in the chamber 52 is maximized upon a high speed low load operating condition of the engine. When the vacuum in the chamber 52 is above a predetermined value upon high speed low load operating condition of the engine, the diaphragm 50 is operated to move against the bias of the spring 62 into the position where it opens the inlet port 61' of the passage 61. The vacuum in the chamber 32 of the pressure regulating avle assembly 30 is therefore reduced by the atmospheric air thus causing a decrease of the vacuum in the chamber 23 of the EGR control valve 20. As a result, the degree of opening of the EGR control valve 20 is reduced whereby the flow rate of recirculated exhaust gases is temporarily modified to a decreased value in this high speed low load operating condition of the engine.
Since the formation of oxides of nitrogen during combustion process of the engine does not take place appreciably when the engine is operated in high speed low load operating condition, the above reduction of the flow rate of recirculated exhaust gases in this operating condition of the engine does not cause any drawback to the emission control effect achieved by the EGR control system, but the combustion efficiency of the engine is improved markedly. As a result, the operating efficiency of the engine, e.g., fuel consumption and operation stability of the engine, is improved.
In this EGR control system as have been thus explained, the spring constant, viz., the ratio of supplied force to resulting deflection, of the spring 29 shall be set to a small value so that the vacuum prevailing in the chamber 23 of the EGR control valve 20 in high load operating condition of the engine can cause the diaphragm 22 to move against the bias of the spring 29 to the position where the EGR control valve 20 provides a predetermined opening degree thereof, because in high load operating condition of the engine the suction vacuum created in the induction passage 10 downstream of the throttle 14 and therefore the vacuum in the chamber 23 of the EGR control valve 20 are reduced to relatively small absolute values.
However, when such a weak spring is employed, the vacuum in the chamber 23 in low speed low load operating condition of the engine shall be set to a value approximately equal to zero, viz., the pressure in the chamber 23 shall be set to a value approximately equal to the atmospheric pressure, so that the EGR control valve 20 with such a weak spring 29 can close sealingly and assuredly in low speed low load operating condition of the engine. Furthermore, when such a weak spring is employed, the vacuum in the chamber 23 corresponding to high speed low load operating condition of the engine cannot be set to a large value because the bias of such a weak spring can be overcomed by a relatively small counter force.
For this reason, the variable range of the vacuum in the chamber 23 between maximum and minimum values is compelled to be set narrow.
This narrow variable range of the vacuum in the chamber 23 causes such drawbacks of the EGR control system that the operation of the EGR control valve 20 tends to be unstable and the magnitude of the vacuum to open the relief valve 48 is likely to fluctuate over a wide range relative to the variable range of the vacuum in the chamber 23. This results in deterioration of the accuracy of EGR control and therefore lack of the reliability of the EGR control system.
This EGR control system further encounters a drawback of poor responsiveness that upon deceleration of a vehicle vehicle with release of the accelerator pedal it takes a rather long response time until the EGR control system discontinues the exhaust gas recirculation. This is due to the fact that the vacuum having prevailed in the chamber 23 of the EGR control valve 20 remains therein for a relatively long time, viz., it takes a relatively long time to dilute the vacuum in the chamber 23 below a predetermined value.
It is accordingly an object of the present invention to improve an EGR control system of the foregoing type.
It is another object of the present invention to provide an improved EGR control system in which the variable range of the vacuum effective to operate an EGR control valve incorporated therein is enlarged to a desirably wide range whereby the operation of the EGR control system becomes stable.
It is a further object of the present invention to provide an improved EGR control system in which a relief valve incorporated therein operates with an increased accuracy in response to a predetermined high speed low load operating condition of the engine.
It is a still further object of the present invention to provide an improved EGR control system which discontinuous the exhaust gas recirculation with an improved responsiveness in a predetermined high speed low load operating condition of the engine.